1. Field of the Invention
This invention lies in the area of rate responsive cardiac pacemakers and the method of operation of same and, more particularly, dual chamber rate responsive pacemaker systems with means for adjusting the rate response relative to the sensed natural sinus rate so as to optimize tracking of the natural sinus rate as long as it is reliable.
2. Description of the Prior Art
Dual chamber rate responsive pacemakers are now widely available from pacemaker manufacturers. Such pacemakers may be of many types, including the types designated as DDDR or VDDR. The DDDR pacemaker paces and senses in both chambers, i.e., both the atrium and the ventricle, and has rate responsive (R) backup to provide pacing in the absence of sensed natural beats, or to provide rate responsive ventricular pacing during atrial tachyarrhythmias in the absence of natural ventricular beats. The DDDR pacemaker has a lead that is placed within the atrium to deliver atrial pace pulses, as well as to sense natural atrial (sinus) signals, and a ventricular lead for pacing and sensing in the ventricle. In contrast, the VDDR pacemaker paces only in the ventricle, although it senses in both the atrium and the ventricle. The VDDR pacemaker system may be made simpler by incorporating a single lead, which has a floating atrial electrode for sensing atrial signals, in a known manner. The VDDR pacemaker is indicated for patients who are determined to have a good and reliable sinus rate, so that for a good bit of the anticipated lifetime of the patient, natural atrial signals will be present from which ventricular pace pulses can be tracked, thereby providing synchronized pacing.
Particularly with respect to the VDDR pacemaker, it is seen that there is an inherent desirability of maximizing use of the sinus rate, i.e., avoiding takeover of the pacing function as long as a good atrial signal is present and sensed. For a rate responsive pacemaker, this leads to the desirability of correlating the rate response as closely as possible to the sensed natural sinus, while still enabling the rate response to take over pacing control when and if the sinus does not accurately reflect cardiac demand. Thus, for example, if a patient develops chronotropic incompetence after implant of a VDDR pacemaker, there must be an ability to switch modes or otherwise enable the sensor response to override the sinus.
The prior art indicates many schemes for adjusting rate response as a function of exercise. Such schemes include algorithms for ramping up rate upon the onset of exercise, and ramping down rate after exercise, so as to provide a more physiologically natural response to exercise. Such pacemaker systems thus involve predetermined programming for optimizing the pacing response in terms of known optimum responses. It is also known to adjust the rate response as a function of a separate sensor, and to switch to the rate response mode if the atrial rate looks to be unreliable. See, for example, U.S. Pat. No. 4,527,568, Rickards, assigned to the same assignee as this invention, where atrial and sensor rate are compared, and the sensor takes over if conditions indicate that this is desirable. However, in pacemakers to date, there is no capability of enabling the rate response function (or algorithm) to track or adapt to the actual sinus rate. Particularly for VDDR mode pacers there is a need to provide a rate response control wherein the sensor-indicated pacing rate is correlated to and tracks just below the sinus rate, so as to optimize sensing of the atrial rate and provide for pacing takeover only when the sinus is unreliable (e.g., atrial tachyarrhythmias) or missing.